The new ValidateDiffCommitResource is capable of validating the diffs

and metadata of a simple linear history (that is, one where each

commit's diff will apply cleanly to the tracking branch). This new

resource was created because the the commit metadata also has to be

verified and the desired behaviour is different enough from the

existing ValidateDiffResource that it warrants creating a new one

(otherwise the existing ValidateDiffResource would become needlessly

complicated).
Previously, DiffCommits created via the DiffCommitManager's methods

were created even if the save parameter was set to False. This is

now no longer the case.
Fix a flake8 violation in the DiffSetManager.

Ran unit tests.
Able to upload a review request with history and have its commits

validated.